1. Field of the Invention
This invention relates to a method for removing heavy metal from activated sludge.
2. Description of the Prior Art
With the continued improvement and growth of wastewater treatment facilities, the quantity of activated sludge produced and the problems associated with its disposal have grown commensurably. Moreover, it is now recognized that because of this growth in the quantity of sludge, many of the more commonly used methods of sludge disposal may need to be more stringently controlled or limited in order to prohibit further environmental pollution. Although the total sludge volume is usually less than 1 percent of the total treated sewage stream, it has been estimated that between 25 and 65 percent of the total capital and operating costs for primary and secondary wastewater treatment systems are expended on sludge handling and subsequent disposal. For these reasons, improvements in sludge handling and disposal methods are being actively pursued.
One method of activated sludge disposal used extensively by coastal cities, ocean dumping, has received severe criticism in recent years because of its detrimental effect on the quality of the marine environment. Accordingly these cities are searching for an economic alternative to this sludge disposal method. One option available to them is land spreading. However, the use of land spreading as a final disposal method requires a sludge that is stabilized as well as pasteurized and essentially free of toxic heavy metals. Pasterurization is a reduction in the concentration of pathogenic organisms in the sludge. As used herein, "heavy metals" are those polyvalent metals commonly referred to as the transition elements, including such metals as cadminum, chromium, copper, mercury, nickel, lead and zinc.
Heavy metals are concetrated in the waste sludges of activated sludge processes by primarily two mechanisms, chelation and chemical precipitation. Since high concentrations of heavy metals are detrimental to plant and animal life, these metals must be removed prior to land disposal of the waste sludge.
When metals are concentrated by chelation, an equilibrium exists between the dissolved heavy metals and insoluble organo-metallic complexes. In order to remove the heavy metals from the sludge and make the sludge suitable as a soil amendment material, the equilibrium of the system must be shifted so that there is a net transfer of heavy metals from the insoluble complex to the soluble form. The most common means for achieving this transfer are by acid addition to lower the pH whereby the metals are displaced with hydrogen ions, the addition of a soluble chelation agent whereby the organo-metallic bonds are broken and the metals form stronger complexes with the soluble chelation agents, or a combination of the two. The sewage sludge is then dewatered and the heavy metals are subsequently precipitated from the liquid phase.
This approach to heavy metals removal ignores some of the sludge heavy metal content present as a chemical precipitate. As a result, in many cases the heavy metal content of the dewatered sludge treated in this manner would still exceed governmental guidelines. The quantity of metals removal by this approach is also limited by the completeness of the dewatering step. Because of their inherent compressibility, biologically produced sludges are typically difficult to dewater. Such dewatering requires expensive dewatering equipment and expensive flocculant aids to achieve maximum solids recovery. Even with the most advanced technology, solid concentrations are still limited to about 30 percent. The requirement of large quantities of acid or chelaton agents for solubilization of the organo-metal chelates also places a severe economic burden on the heavy metals removal system. For these reasons, this approach does not offer a practical solution to the problem of heavy metals removal.
Heavy metals may also be concentrated in activated sludges as insoluble hydroxide, carbonate and sulfide precipitates. Because of the typical pH of most wastewater sludges (i.e., between 6 and 8), the amount of heavy metal carbonates and hydroxides formed are minimal. The metal sulfides are of major concern because they are highly insoluble, finely distributed and cannot be separated from the solid biomass. The normal procedure to reduce sulfides is to oxidize the material either chemically or biochemically. Such oxidation converts the insoluble heavy metal sulfides to the soluble sulfate species.
Allen et al, U.S. Pat. No. 3,642,435 describes a method of chemically oxidizing metal sulfides. A metal sulfides-containing ore is finely ground and mixed with water to form a slurry. The slurry is heated to between 175.degree. F. and 250.degree. F. and contacted with an oxygen-containing gas at above atmospheric pressure to convert the metal sulfides into water-soluble metal sulfates. The liquid phase is then recovered and subsequently treated for metals removal.
Duncan et al, U.S. Pat. No. 3,304,353 describes a method of biochemically oxidizing metal sulfides. A metal sulfide-containing ore is finely ground and mixed with water to form a slurry. This slurry is acidified to a pH ranging between 1.5 and 3.0. Bacteria, being recycled from a later stage of the process, and nutrients are added to this acidified slurry of ground ore as it passes to a fermentation tank. The bacteria comprises a relatively pure culture of Thiobacillus ferrooxidans, which is a certain type of bacteria capable of oxidizing sulfide in combination with metals at the recited low pH conditions. In the Duncan et al fermentation tank, air is introduced to provide oxygen for the bacteria and is agitated to facilitate their contacting the sulfide ore. The product of the fermentation tank is passed to a gravity separator which removes the particulate material from the liquor. Depending on the extraction achieved, the particulate material so recovered may be discarded or may be reground prior to further bacterial leaching. The liquid portion of the slurry, still containing a large amount of bacterial cells, may then pass to a bacterial separator or directly to a metal recovery stage where the metal is removed from solution by electrical or chemical deposition. The bacterial cells are preferably recovered and returned to the fermentation tank. In this way, as new batches of minerals are repeatedly brought into contact with the bacteria, the organisms adapt or mutate to achieve maximum utilization of both ferrous iron and sulfide. According to Duncan et al, not only is the rate of metal leaching increased but the time period which elapses before leaching begins is virtually eliminated.
Neither of the aforesaid methods of removing heavy metal sulfides from an ore, however, suggest to one skilled in the wastewater treatment art, the necessary conditions for removing heavy metal sulfides from sewage sludges. Unlike the Allen et al and Duncan et al methods, the heavy metals in a sewage sludge are typically at such low concentrations that the value of the heavy metals recovered does not offset the costs associated with their recovery. For this reason, processes as Allen et al which require considerable heating of a dilute slurry to an elevated temperature cannot be economically justified. In a similar fashion, the cost of acidifying the dilute sludge to a pH in the range of 1.5 to 3.0 as taught by Duncan et al, is also prohibitive.
The Duncan et al process is concerned with cultivating a specific culture of aerobic bacteria capable of oxidizing metal sulfides. These bacteria must be maintained at a pH or between 1.5 to 3.0 to thrive. This condition is not suitable for the growth of the typical microbial cultures normally present in wastewater sludge treatment processes.
In addition to heavy metals removal, one additional requirement in treatment of sewage sludges is sludge volatile solids reduction and, in cases where land spreading is the final means of disposal, pasteurization. Unless the volatile solids are reduced prior to disposal, the sewage sludge will still contain enough biodegradable solids to undergo putrefaction upon setting. The aeration and agitation step of the Duncan et al process would not produce significant volatile solids reduction because of the low pH characteristic of this step. As previously recited, the microbial species capable of reducing the biodegradable portion of sewage sludges by aerobic and anaerobic digestion do not proliferate at low pH conditions. Furthermore, treatment of the dewatered residue of the Duncan et al process by such microbial species is also prohibited for the same reason. Prior to such treatment, the residue must be treated with an appropriate caustic solution to increase the pH and make the sludge amenable to further treatment.
Both of these ore treatment processes also require a dewatering step to separate the soluble heavy metal-containing liquid from the solids. As stated previously, biologically produced sludges, because of their inherent compressibility, are typically difficult to dewater. Such dewatering is expensive and the ultimate solids concentration of the dewatered cake is limited by present echnology to around 30 percent. Obviously, the quantity of heavy metals removed would be limited by the completeness of this dewatering step.
It is an object of this invention provide a method for removal of heavy metals from activated (sewage) sludges which does not require addition of large quantities of chemicals, heating for operation at elevated temperature, or dewatering.
As will be explained in detail hereinafter, this invention accomplishes these objects while simultaneously achieving volatile solids reduction and most instances without requiring significant quantities of chemicals for pH adjustment and other purposes.